Active magnetic ranging techniques are commonly utilized in well twinning and well intercept applications, for example, including steam assisted gravity drainage (SAGD) and coal-bed methane (CBM) drilling applications. In one known active ranging method (e.g., as disclosed in U.S. Pat. No. 5,485,089), a high strength electromagnet is pulled down through a cased target well during drilling of a twin well. An MWD tool deployed in the drill string measures the magnitude and direction of the magnetic field during drilling of the twin well to determine a distance and direction to the target. In another known active ranging method (e.g., as disclosed in U.S. Pat. No. 5,589,775), a magnet is mounted on a rotating sub below a drilling motor (deployed in the twin well). A wireline tool is pulled down through the cased target well and measures the magnitude and direction of the magnetic field during drilling of the twin well. Both methods utilize the magnetic field measurements to compute a range and bearing (a distance and a direction) from the twin well to the target well and to guide continued drilling of the twin.
The prior art active ranging methods described above, while utilized in commercial SAGD operations, are known to include several significant drawbacks. For example, both techniques require precise lateral (z-directional) alignment between the magnetic source deployed in one well and the magnetic sensors deployed in the other. Misalignment can result in a misplaced twin well, which can have a significant negative impact on future well productivity. Moreover, the steps taken to assure proper alignment (such as making magnetic field measurements at multiple longitudinal positions in one of the wells) are time consuming (and therefore expensive) and may further be problematic in deep wells. Still further, the approach described in the '089 patent requires surveying measurements to be made at both positive and negative electromagnetic source polarities in order to cancel out remanent magnetization in the target casing. As a result, surveying time (and therefore the time required to drill the twin well) becomes even more excessive.
U.S. Pat. Nos. 6,985,814; 7,538,650; 7,617,049; 7,656,161; and 7,712,519 disclose enhanced passive ranging techniques suitable for well twinning and well intercept applications. These techniques often impart certain advantages over the above described active ranging techniques. However, magnetizing large numbers of casing tubulars, storing the magnetized tubulars, and deploying the magnetized tubulars in the target well tends to introduce technical and logistical challenges. While these challenges have been adequately overcome for commercial deployment of the technology, there is a need for an improved method of magnetizing the target well, particularly a method that reduces handling requirements of the magnetized tubulars.